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aggregate impact of these is not yet known and indeed many
such impacts and their interactions have not yet been reliably
quantified, especially at the global scale. An increase in mean
temperature can be confidently expected, but the impacts on
productivity may depend more on the magnitude and timing of
extreme temperatures. A mean sea level rise can also be con-
fidently expected, which could eventually result in the loss of
agricultural land through permanent inundation. The impacts of
temporary flooding through storm surges may be large although
less predictable.
Freshwater availability is critical, but predictability of pre-
cipitation is highly uncertain and there is an added problem of
lack of clarity on the relevant metric for drought—some studies
including IPCC consider metrics based on local precipitation
and temperature such as the Palmer drought severity index, but
this does not include all relevant factors. Agricultural impacts
in some regions may arise from climate changes in other
regions, owing to the dependency on rivers fed by precipitation,
snowmelt and glaciers some distance away. Drought may also
be offset to some extent by an increased efficiency of water
use by plants under higher CO
2
concentrations, although the
impact of this is again uncertain especially at large scales. The
climate models used here project an increase in annual mean
soil moisture availability and run-off in many regions, but nev-
ertheless, across most agricultural areas there is a projected
increase in the time spent under drought as defined in terms of
soil moisture.
Moreover, the sign of crop yield projections is uncertain as
this depends critically on the strength of CO
2
fertilisation and
also O
3
damage. Few studies have assessed the response of crop
yields to CO
2
fertilisation and O
3
pollution under actual grow-
ing conditions, and consequently model projections are poorly
constrained. Indirect effects of climate change through pests
and diseases have been studied locally, but a global assessment
is not yet available. Overall, it does not appear to be possible at
the present time to provide a robust assessment of the impacts
of anthropogenic climate change on global-scale agricultural
productivity.
References
Abrol, Y. P., Bagga, A. K., Chakravorty, N. V. K. and
Wattal, P. K. 1991. Impact of rise in temperature on the
productivity of wheat in India. In:
Impact of Global Climate
